1. Field of the Invention
This invention is in the field of pharmacology, and relates to drug dosage forms that are designed to deliver drugs concurrently in both immediate-release and prolonged-release delivery profiles.
2. Description of the Prior Art
Certain pharmacological therapies either require or benefit from the sequential administration of two or more drugs. While this can be accomplished if the administration is done according to a strict time schedule, many patients have difficulty complying with such a schedule without the assistance of a medical professional. Some therapies involve only an immediate but rapidly declining high-level dosage and a prolonged dosage at a low or moderate level, the two dosages being either of the same drug or of a different drug. Even two-dosage therapies such as these however can be troublesome to maintain if a separate administration is needed for each dosage. Certain pharmaceutical formulations have therefore been developed that combine both functions into a single dosage form. This simplifies the therapy and reduces or eliminates the chances of improper administration.
Many unitary dosage forms have been proposed that combine an immediate release drug with a prolonged release drug by placing the drugs in different layers of a tablet or by placing one drug in a quickly-dissolving or quickly-dispersing shell that surrounds the slowly dissolving or swellable core that contains the other drug. The immediate-release layer or shell is typically formed by coating the pre-formed prolonged-release portion with a solution or a suspension of the immediate-release drug and a polymer and then allowing the liquid carrier in which the drug and polymer are dissolved or suspended to evaporate, leaving a solid dry outer layer. In some cases, however, the liquid carrier tends to cause swelling of the outer surface of the prolonged-release portion. As the prolonged-release matrix swells, some of the drug intended for immediate release enters the prolonged-release matrix. When the immediate-release drug is applied as a suspension of particles, the particles tend to become trapped in the prolonged-release matrix. High-potency drugs that are intended for immediate release are therefore retained rather than released. The problem can be particularly acute when the drug is insoluble or of low solubility, since unintended retention of the drug in the prolonged-release portion can significantly reduce the amount of the drug that is available for absorption into the patient's bloodstream.